.TH btrfsslower 8  "2016-02-15" "USER COMMANDS"
.SH NAME
btrfsslower \- Trace slow btrfs file operations, with per-event details.
.SH SYNOPSIS
.B btrfsslower [\-h] [\-j] [\-p PID] [min_ms] [\-d DURATION]
.SH DESCRIPTION
This tool traces common btrfs file operations: reads, writes, opens, and
syncs. It measures the time spent in these operations, and prints details
for each that exceeded a threshold.

WARNING: See the OVERHEAD section.

By default, a minimum millisecond threshold of 10 is used. If a threshold of 0
is used, all events are printed (warning: verbose).

Since this works by tracing the btrfs_file_operations interface functions, it
will need updating to match any changes to these functions.

Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
\-p PID
Trace this PID only.
.TP
min_ms
Minimum I/O latency (duration) to trace, in milliseconds. Default is 10 ms.
.TP
\-d DURATION
Total duration of trace in seconds.
.SH EXAMPLES
.TP
Trace synchronous file reads and writes slower than 10 ms:
#
.B btrfsslower
.TP
Trace slower than 1 ms:
#
.B btrfsslower 1
.TP
Trace slower than 1 ms, and output just the fields in parsable format (csv):
#
.B btrfsslower \-j 1
.TP
Trace all file reads and writes (warning: the output will be verbose):
#
.B btrfsslower 0
.TP
Trace slower than 1 ms, for PID 181 only:
#
.B btrfsslower \-p 181 1
.TP
Trace for 10 seconds only:
#
.B btrfsslower \-d 10
.SH FIELDS
.TP
TIME(s)
Time of I/O completion since the first I/O seen, in seconds.
.TP
COMM
Process name.
.TP
PID
Process ID.
.TP
T
Type of operation. R == read, W == write, O == open, S == fsync.
.TP
OFF_KB
File offset for the I/O, in Kbytes.
.TP
BYTES
Size of I/O, in bytes.
.TP
LAT(ms)
Latency (duration) of I/O, measured from when it was issued by VFS to the
filesystem, to when it completed. This time is inclusive of block device I/O,
file system CPU cycles, file system locks, run queue latency, etc. It's a more
accurate measure of the latency suffered by applications performing file
system I/O, than to measure this down at the block device interface.
.TP
FILENAME
A cached kernel file name (comes from dentry->d_name.name).
.TP
ENDTIME_us
Completion timestamp, microseconds (\-j only).
.TP
OFFSET_b
File offset, bytes (\-j only).
.TP
LATENCY_us
Latency (duration) of the I/O, in microseconds (\-j only).
.SH OVERHEAD
This adds low-overhead instrumentation to btrfs writes and fsyncs, as well
as all system reads and opens (due to the current implementation of the
btrfs_file_operations interface). Particularly, all reads and writes from
the file system cache will incur extra overhead while tracing. Such reads and
writes can be very frequent (depending on the workload; eg, 1M/sec), at which
point the overhead of this tool may become noticeable.
Measure and quantify before use. If this
continues to be a problem, consider switching to a tool that prints in-kernel
summaries only, such as btrfsdist(8).
.PP
Note that the overhead of this tool should be less than fileslower(8), as
this tool targets btrfs functions only, and not all file read/write paths
(which can include socket I/O).
.SH SOURCE
This is from bcc.
.IP
https://github.com/iovisor/bcc
.PP
Also look in the bcc distribution for a companion _examples.txt file containing
example usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Brendan Gregg
.SH SEE ALSO
btrfsdist(8), biosnoop(8), funccount(8), fileslower(8)
